The overall objective of this project is to develop a new block detector module for multi-layer, high resolution positron emission tomography that provides high resolution over the entire imaging field, high maximum counting rates, and high sensitivity. The advantages over existing block detector designs is a smaller block size, which results in higher maximum coincident event rates, and individual coupling of scintillator crystals to position sensitive photodiodes, which improves the spatial resolution. Special features of this approach are: Individual readout of narrow, heavy-atom scintillation crystals for 2 mm spatial resolution, maximum efficiency, and high maximum event rates. Position-sensitive solid-state photodetectors for identifying the crystal of interaction and measuring depth of interaction, thus reducing the position error that causes radial blurring. This detector design is being developed for the quantitative measurement of tracer compounds with high spatial and temporal resolution to measure specific perfusion and metabolism. This project proposes only to do proof of principle studies culminating with test table images - no complete tomographs will be constructed. The design, when completed, will be offered to potential collaborators in private industry (CTI, Inc. of Knoxville, TN has already shown interest in this design) who may incorporate it into complete scanners. We believe that the detector module proposed can be used to construct either of the following specific tomographs: A human brain imaging tomograph with 2 nun crystals and 2 mm resolution over the entire brain. This tomograph would permit the study of cortical and brain stem biochemistry with sufficient resolution to provide accurate quantitation in 4 x 4 x 5 mm3 volumes. An animal tomograph with 1 mm crystals and 1-1.5 mm resolution. This system would be able to image small animals with unprecedented resolution and approach the goal of in-vivo autoradiography, in which the animals are used as their own controls.